In recent years, in office automation (OA) equipment, such as word processors, personal computers, facsimile machines, various measuring instruments, such as medical measuring instruments, and other devices, ink jet printers have been extensively used for printing information from these devices at a high density. As well known in the art, in the ink jet printer, an ink droplet is ejected from a head section of the printer and deposited directly onto a recording medium, such as recording paper, to perform monochrome or color printing. The ink jet printer has many advantages including that printing can be performed on even a three-dimensional recording medium, running cost is low since plain paper can be used as the recording medium, the head can be simply loaded, the need to provide the step of transfer, fixation and the like can be eliminated, color printing is easily performed, and a sharp color printed image can be provided. The head section of the ink jet printer can be classified into several types according to the method for ejecting ink droplets from the head section. Among them, a typically and advantageously used one is a piezoelectric ink jet head.
The piezoelectric ink jet head generally comprises: a plurality of ink chambers which are disposed at equidistant spaces and function as an ink flow passage and a pressurizing chamber for ejecting an ink; and a nozzle plate mounted on the front end of the ink chambers and equipped with nozzles, for ejecting an ink, corresponding respectively to the ink chambers; and pressurizing means for pressurizing an ink within the ink chamber in response to a demand for printing. The pressurizing means comprises a piezoelectric element, and an electrostrictive effect attained by this piezoelectric element is utilized to create a pressure wave within the ink chamber, filled with an ink, in the head section, permitting the ink to be ejected through the nozzle in the head section.
Ferroelectric elements have been extensively used as a piezoelectric element in the above ink jet head or as, for example, capacitors, actuators, memories, and other elements. A ferroelectric element consists essentially of a ferroelectric body or a ferroelectric material. Examples of typical ferroelectric materials include lead zirconate titanate (PZT) generally represented by Pb(Zr,Ti)O.sub.3, (Pb,La)(Zr,Ti)O.sub.3 (PLZT), and Pb(Mg.sub.1/3 Nb2/3)O.sub.3 (PMN). In particular, it is known that ferroelectrics containing lead (Pb) as one metal component, including PZT, have large remanence, specific permittivity, and piezoelectric constant and possess excellent piezoelectricity and ferroelectricity. In the present specification, the ferroelectric material will be described particularly with reference to PZT.
The above ferroelectric elements, particularly thin film elements of PZT, have hitherto been produced by various film forming methods, such as sputtering, sol-gel process, CVD, and laser ablation, or methods related thereto. When the thin film element is formed particularly in a large thickness of 10 to 20 .mu.m, a method has been used wherein the film thickness is increased by prolonging the film formation time or by repeating the film formation procedure. Further, when a PZT element having a perovskite structure is produced, firing is generally performed in a high temperature atmosphere of 500 to 800.degree. C.
Among the film formation methods, the sol-gel process which is particularly included in the range of a solution preparation method is advantageous in that a high-purity thin film of PZT can be formed, a starting material can be quantitatively dissolved in a solution and, hence, the composition of the formed thin film of PZT can reflect the composition of the starting material used, which facilitates the control of the composition and can provide a thin film of PZT having high surface smoothness by repetition of spin coating and firing. The solvent used in the preparation of a sol-gel solution is in many cases an alcohol solvent because the metal for PZT takes the form of a metal alkoxide or a metal salt of an organic carboxylic acid. The solution prepared by the sol-gel process may be coated onto a substrate, for example, by spin coating or dip coating to form a film. In this film formation, addition of a photosensitive resin to the solution enables patterning by photoetching.
More specifically, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-112550 discloses a method for forming a thin film of PZT which comprises hydrolyzing a metal alkoxide as a PZT material to prepare a sol solution, adding a soluble organic polymer, for example, polyethylene glycol monomethyl ether, to the solution and thoroughly stirring the solution. Subsequently, a platinum electrode is formed on a silicon substrate, a sol solution prepared above is spin-coated on the electrode, and the coating is heated to about 350.degree. C. The prefiring results in the formation of a porous thin film of a gel. The same starting material as the above PZT material is hydrolyzed to form a sol solution. In this case, however, no polyethylene glycol monomethyl ether is added. The sol solution is spin-coated onto the above porous thin film of a gel to form a coating which is then dried by heating at 400.degree. C. The resultant thin film is fired in an oxygen atmosphere for 15 hr. The firing temperature is generally 600 to 700.degree. C. Thus, a thin film of PZT having a perovskite structure can be formed through a series of steps. In this film formation method, a sol is filled into pores of the porous thin film of a gel. This reduces the porosity and can offer high Young's modulus and consequently excellent electric properties. Further, since the size of the pore is not more than 1 .mu.m, no cracks are created.
Japanese Unexamined Patent Publication (Kokai) No. 6-119811 discloses a process for producing a ferroelectric thin film element, comprising producing a ferroelectric thin film by a sol-gel process using a metal alkoxide as a main starting material, wherein particles of a ferroelectric oxide are added to a sol prepared by hydrolyzing the starting material followed by homogeneous mixing to prepare a coating liquid. In this film formation method, addition of ferroelectric oxide particles to the sol enables a thick film to be easily formed and, in addition, results in the formation of a thin film of PZT having excellent properties and electric characteristics.
When all the above prior art methods are taken into consideration, it can be said that, in the conventional methods for producing a ferroelectric element, it is very difficult to produce a ferroelectric thin film element having a relatively large thickness of 1 .mu.m or more. Further, according to a sol-gel process which can generally provide a film of a submicron thickness, repetition of coating of a coating solution prepared by the sol-gel process can provide a thick film. In this case, however, it is difficult to form a dense film having excellent qualities.
Further, the formation of the thick film poses a problem that cracking is often created to make it impossible to provide ferroelectric properties. Specifically, for example, coating of an aqueous PZT precursor solution by a conventional method, such as dip coating or spin coating, followed by drying, degreasing and firing to form a thin film of PZT having a thickness of not less than 1 .mu.m often results in the creation of cracking. The creation of cracking could not be avoided even when the thin film of PZT is formed by stacking a plurality of thinner films on top of another. Creation of cracking in the thin film of PZT results in lowered film density, makes it impossible to form an element, such as an electrode, on the top surface of the film, and, hence, makes it impossible to utilize the thin film of PZT, for example, as a piezoelectric element of an ink jet head. In fact, since the film thickness of the piezoelectric element used in the head of the ink jet printer is generally about 10 to 20 .mu.m, the conventional methods are unsatisfactory also from the practical viewpoint.
Referring again to Japanese Unexamined Publication (Kokai) No. 6-119811, as described above, it teaches that, in the preparation of a thin film of PZT by the sol-gel process, after the preparation of a PZT sol-gel solution, separately prepared fine particles of PZT are mixed with the PZT sol-gel solution to prepare a homogeneous coating liquid. This method, however, is troublesome because the PZT sol-gel solution and the fine particles of PZT to be added to the PZT sol-gel solution should be prepared separately. Further, a PZT sol-gel solution is prepared by the method which is different from the method by which the fine particles of PZT are prepared. Therefore, in the addition of the fine particles to the sol-gel solution, it is difficult to achieve homogeneous dispersion.
Preparation of a ferroelectric element through a green sheet of a precursor to the ferroelectric element has also been extensively carried out in the art. In this preparation method, in general, a fine powder of a ferroelectric material is mixed with a suitable binder material and a solvent, and the resultant mixed liquid is formed into a sheet by using a coating device, such as a doctor blade or a roll coater, or screen printing. The green sheet is fired at 1000.degree. C. or above, and a conductive paste is coated thereon, followed by firing at the firing temperature of the conductive paste. In this method, however, the adoption of a high temperature of 1000.degree. C. or above as the firing temperature poses a problem that elements constituting the dielectric are unfavorably evaporated during firing, leading to a variation in composition ratio in the resultant ferroelectric and, hence, rendering the control of the composition ratio difficult. The high-temperature firing poses an additional problem that the electrode material to be coated onto a green sheet for developing the ferroelectricity and the piezoelectricity is limited to those having heat resistance.